Delta-sigma analog-to-digital converters provide a digital output with excellent signal to noise ratio characteristics. The typical delta-sigma analog-to-digital converter comprises a delta-sigma modulator which receives an analog input signal and produces an oversampled digital signal which is filtered by a digital filter. A loop filter inside the delta-sigma modulator shapes the quantization noise typically into a high pass characteristic; that is, the noise is very low at low frequencies and very high at high frequencies. The digital filter provides low pass filter characteristics to ideally eliminate most of the quantization noise in the high frequencies.
In practical delta-sigma modulators, any operations which result in coupling a non-linear function of the quantization noise into the analog input must be carefully avoided. Such non-linearities can degrade the noise performance at frequencies in the band of interest. For example, the quantization noise component at frequency f.sub.s /2-.DELTA. can be modeled as EQU g(kT)=a cos [2.pi.(f.sub.s /2-.DELTA.)kT] (1)
A square law non-linearity produces: ##EQU1## Since f.sub.s =1/T, ##EQU2## This equation contains a component at frequency 2.DELTA.. Thus, non-linearities in a delta-sigma modulator system have the risk of taking high amplitude noise near f.sub.s /2 and translating it down into the band of interest where the digital filter provides no attenuation.
Therefore it can be appreciated that a method for transmission of output data from a delta-sigma modulator which avoids injecting noise into the frequency band of interest is highly desirable.